This invention relates generally to electric propulsion systems for traction vehicles and, more particularly, to a method and apparatus for automatically adjusting maximum allowable propulsion speed to enable the vehicle to be operated with optimum productivity.
An electric propulsion system for a traction vehicle, such as a large haulage truck, typically comprises a prime mover-driven electric generating means for supplying electric power to a pair of high-horsepower electric traction motors respectively connected in driving relationship to a pair of wheels on opposite sides of the vehicle. The prime mover is commonly a diesel engine, and the traction motors are generally adjustable speed, reversible direct current (d-c) electric motors. A vehicle operator controls the vehicle speed and direction of travel, i.e., forward or reverse, by manipulation of a speed control pedal and a forward-reverse selector switch. This speed control pedal is adapted to control the engine speed (rpm) which determines the power output of the generating means, thus varying the magnitude of the voltage applied to the traction motors.
Deceleration of a moving vehicle is accomplished by releasing the speed control pedal and either allowing the vehicle to coast or activating its mechanical brakes or electrical retarding system. In the electrical retarding mode of operation (sometimes called electric or dynamic braking), the motors behave as generators, and the magnitude of the voltage generated across the armature windings of each motor is proportional to the rotational speed and the field excitation current of the motor. Dynamic braking resistor grids are connected across the armatures of the respective motors to dissipate the electric power output of the motors during electrical retarding. The average magnitude of current in each resistor grid is a measure of the retarding effort of the associated motor.
It is common to establish a maximum allowable propulsion speed for such vehicles. Conventional practice is to include a preset overspeed limit in the controls of the propulsion system. Several factors affect the selection of the speed limit. It can not be higher than the maximum safe speed for entering any downhill grade of the roadway along which the vehicle will travel. The maximum safe entry speed is the highest constant speed that can be maintained on the downhill grade with electrical retarding in effect. If the actual entry speed were higher than this maximum, the available dynamic retarding effort of the traction motors would be insufficient to keep the vehicle from accelerating (a "runaway" condition). The maximum retarding ability of d-c traction motors depends primarily on the commutation limit of the motors. Above the commutation limit, electrical arcs or sparks can occur, with resulting damage to the motor commutator and brushes. The commutation limit is a function of armature current magnitude multiplied by armature velocity. At high speeds, the current must be kept relatively low in order to avoid such arcing, thereby resulting in lower available dynamic retarding effort. If the available retarding effort were insufficient to slow the vehicle, service brakes could be used. However, at speeds above about five miles per hour service brakes should not be used because of their undesirably rapid wear at such speeds.
The retarding effort required to slow the traction vehicle is a function of the weight of the vehicle, including any payload carried by it, and the slope of the grade on which the vehicle is traveling. Dynamic retarding effort in a d-c motor is essentially the product of armature current and field generated flux. If armature current is regulated to a high magnitude in order to generate sufficient retarding effort to slow a fully loaded vehicle, the speed of the vehicle must be very low to avoid arcing at the commutator and brushes.
Prior attempts to ensure safe operation of a haulage vehicle without risking damage to its traction motors have limited the maximum propulsion speed in accordance with worst possible conditions, i.e., an overspeed limit is set for the steepest downhill grade and heaviest payload that are expected to be encountered by the vehicle in normal operation. In some instances, a manual switch has been placed in the vehicle cab to allow the operator to manually select an overspeed limit for either a loaded or an empty vehicle. Manual systems are generally unsatisfactory since they are subject to human error and forgetfulness. Limiting the overspeed setting to worst case conditions prevents the vehicle from moving at higher speeds that would be both desirable and permissible when the vehicle is empty or traveling on a level roadway, thereby reducing the vehicle's productivity.